mcpinput.py

#!/usr/bin/python

import smbus
import sys
import time

bus = smbus.SMBus(1)
address = 0x20
bus.write_byte_data(address,0x00,0x00) # set A7-A0 as output pins                          
bus.write_byte_data(address,0x01,0x07) # set B0-B3 as input pins , B4-7 as output 0000111
bankA=0x12
bankB=0x13

def main():
    a = 0
    b = 0
    inp = 0
    delay = 0.05
    #Turnoff all outputs in bank A
    a = 255    # 11111111  
    bus.write_byte_data(address,bankA,a)
    
    #Reset all buttons
    #you need to bring them up using a 10KOhm resistor connected to 3.3v
    #You can't do the following using code and expect it to be stable
    #If you do the following it will be unpredictable.
    #b = 255  # 11111111    
    #bus.write_byte_data(address,bankB,b)
    
    while True:
        # read input from bank B and store to "inp" variable
        inp = bus.read_byte_data(address,bankB)
       
        #Check if button B0 is pressed, light relay A6
        if inp == 254:  # 11111110 (B0)
            #print "B7"
            bus.write_byte_data(address,bankA,191) #light relay A6 10111111
                
        #Check if button B1 is pressed, light relay A7
        if inp == 253:  # 11111101 (B1)
            #print "B6"
            bus.write_byte_data(address,bankA,127) #light relay A7 01111111
            
        #Check if button B2 is pressed, light relay A6
        if inp == 251:  # 11111011 (B2)
            #print "B5"
            bus.write_byte_data(address,bankA,255) #turn off all relay A7 11111111
         
        time.sleep(delay)
        
        #print str(inp)


      
if __name__ == "__main__":
    main()